Making an electrode for a conductivity meter

Hi

I am trying to make a conductivity meter and would like to know if any body
can advise on how to make the electrodes? Do I make them from paladium or
can I use something like alluminum?
Also, would I get an improved conductivity value using an AC signal?

Cheers

Wayne

Hello,

1- It exist conductivity meters in the market. You can just buy one
of them
: you'll save your time and money.

2- If you really want to make one by yourself for pratice, then :
* conductivity cells usually consist of two 1cm2 plates of platinized

platinum correctly embedded
in insulator and separated from each other by 1cm.

* measuring signal is AC to avoid problems linked with mass transfer
limitations when a DC current is applied that will add overvoltages
that
will distort the measurement.

* platinum is used in standard cells, despite its high cost, because
it has
very low charge transfer resistance. Charge transfer resistance adds
to the
ohmic resistance you want to measure and thus distort the result.
* aluminum is covered by a layer of Al2O3 that will distort your
measurement, because the electrons will need to pass this layer.

n2mp wrote:
Hello,

1- It exist conductivity meters in the market. You can just buy one
of them
: you'll save your time and money.

Very true.

2- If you really want to make one by yourself for pratice, then :
* conductivity cells usually consist of two 1cm2 plates of platinized

platinum correctly embedded
in insulator and separated from each other by 1cm.

This is a "standard" conductivity cell - you can make the plates bigger
and closer together if you want to see a lower resistance when
measuring the conductivity of a particular solution. Making them
smaller and further apart lets you see a higher resistance, but you end
up averaging the conductivity over a range of paths through the liquid,
and the maths can get complicated.

* measuring signal is AC to avoid problems linked with mass transfer
limitations when a DC current is applied that will add overvoltages
that
will distort the measurement.

You really have to measure conductivity with an AC signal - you get
additional voltage drops at the electrode surfaces in addition to the
resistive losses through the bulk of the liquid, and you have to
minimise these additional losses if your measurement is going to say
anything useful about the conductivity of the solution. Even with an AC
signal, you've always got a capacitative impedance at the electrodes in
series with the resistance of the solution - a higher frequencies this
can be just the "double layer capacitance" but at lower frequencies you
can also see the Warburg impedance which looks like a frequency
dependent capacitance in series with a frequency dependent resistance -
see

http://www.gamry.com/App_Notes/EIS_Primer/EIS_Primer.pdf

* platinum is used in standard cells, despite its high cost, because
it has
very low charge transfer resistance. Charge transfer resistance adds
to the
ohmic resistance you want to measure and thus distort the result.
* aluminum is covered by a layer of Al2O3 that will distort your
measurement, because the electrons will need to pass this layer.

Platinum is used in standard cells because it doesn't corrode, and
because it is easy to electroplate a layer of "platinum black" onto a
platinum surface, which vastly increases the effective surface area and
reduces the voltage drops at the electrode surfaces in the same
proportion. Carbon is often used in big cells where cost is a problem,
but it is much more difficult to work with.

Stainless steel is just as bad as aluminium - it is "stainless" because
the surface is protected by a thin layer of Cr2O3 and I've had
conductivity measurements wrecked by the additional resistance of this
layer (which changes with the chemical nature of the solution being
measured). My bosses were not pleased with having to pay for platinum
rather than stainless steel, but should have been more upset about ever
having wasted time on stainless steel electrodes in the first place.
-------
Bill Sloman, Nijmegen

* conductivity cells usually consist of two 1cm2 plates of platinized
platinum correctly embedded

n2mp wrote:

* conductivity cells usually consist of two 1cm2 plates of platinized
platinum correctly embedded

I have always used pure Gold plating for conductivity meter electrodes.
Do you have any information as to why Platinum would be prefered?

Thanks Bill
Do you know where I could buy a few of these 1cm^2 platinum
plates. My research is taking on a great deal of equipment and I will be
modifying and customising the equipment that is why I can not just buy one.

n2mp wrote:

* conductivity cells usually consist of two 1cm2 plates of platinized
platinum correctly embedded

I have always used pure Gold plating for conductivity meter electrodes.
Do you have any information as to why Platinum would be prefered?

John Larkin wrote:

On Wed, 29 Dec 2004 18:22:54 +0000, Guy Macon
http://www.guymacon.com/> wrote:


n2mp wrote:

* conductivity cells usually consist of two 1cm2 plates of platinized
platinum correctly embedded

I have always used pure Gold plating for conductivity meter electrodes.
Do you have any information as to why Platinum would be prefered?

Wouldn't a potted torroidal inductor make an interesting conductivity
sensor? No contacts to crud up.

It certainly would. And they are a commercial product. They contain
two toroidal cores, axially aligned. The winding on one produces an
EMF in the liquid, driving a current that passes through the hole.
The winding on the second one acts as a current transformer, measuring
that current.

On Wed, 29 Dec 2004 18:22:54 +0000, Guy Macon
http://www.guymacon.com/> wrote:


n2mp wrote:

* conductivity cells usually consist of two 1cm2 plates of platinized
platinum correctly embedded

I have always used pure Gold plating for conductivity meter electrodes.
Do you have any information as to why Platinum would be prefered?

Wouldn't a potted torroidal inductor make an interesting conductivity
sensor? No contacts to crud up.

On Wed, 29 Dec 2004 15:46:37 -0500, John Popelish <jpopelish_at_rica.net
wrote:

It certainly would. And they are a commercial product. They contain
two toroidal cores, axially aligned. The winding on one produces an
EMF in the liquid, driving a current that passes through the hole.
The winding on the second one acts as a current transformer, measuring
that current.

That's bizarre. Why not a single torroid?

John Larkin wrote:

On Wed, 29 Dec 2004 15:46:37 -0500, John Popelish <jpopelish_at_rica.net
wrote:

It certainly would. And they are a commercial product. They contain
two toroidal cores, axially aligned. The winding on one produces an
EMF in the liquid, driving a current that passes through the hole.
The winding on the second one acts as a current transformer, measuring
that current.

That's bizarre. Why not a single torroid?

I think the two core approach makes it easier to separate the small
current transformer currents from the large excitation current.

http://vfaut.axelero.net/honeywell/5000tc.pdf

On Wed, 29 Dec 2004 15:46:37 -0500, John Popelish <jpopelish_at_rica.net
wrote:

John Larkin wrote:

On Wed, 29 Dec 2004 18:22:54 +0000, Guy Macon
http://www.guymacon.com/> wrote:


n2mp wrote:

* conductivity cells usually consist of two 1cm2 plates of platinized
platinum correctly embedded

I have always used pure Gold plating for conductivity meter electrodes.
Do you have any information as to why Platinum would be prefered?

Wouldn't a potted torroidal inductor make an interesting conductivity
sensor? No contacts to crud up.

It certainly would. And they are a commercial product. They contain
two toroidal cores, axially aligned. The winding on one produces an
EMF in the liquid, driving a current that passes through the hole.
The winding on the second one acts as a current transformer, measuring
that current.

That's bizarre. Why not a single torroid?

That was my initial reaction. But calculating the conductivity from the

I read in sci.electronics.design that John Larkin <jjlarkin_at_highSNIPland
THIStechPLEASEnology.com> wrote (in <ok56t0hb8n0a6kdvlu4hqt39vof9bd80os@
4ax.com>) about 'Making an electrode for a conductivity meter', on Wed,
29 Dec 2004:
On Wed, 29 Dec 2004 15:46:37 -0500, John Popelish <jpopelish_at_rica.net
wrote:

John Larkin wrote:

On Wed, 29 Dec 2004 18:22:54 +0000, Guy Macon
http://www.guymacon.com/> wrote:


n2mp wrote:

* conductivity cells usually consist of two 1cm2 plates of platinized
platinum correctly embedded

I have always used pure Gold plating for conductivity meter electrodes.
Do you have any information as to why Platinum would be prefered?

Wouldn't a potted torroidal inductor make an interesting conductivity
sensor? No contacts to crud up.

It certainly would. And they are a commercial product. They contain
two toroidal cores, axially aligned. The winding on one produces an
EMF in the liquid, driving a current that passes through the hole.
The winding on the second one acts as a current transformer, measuring
that current.

That's bizarre. Why not a single torroid?

That was my initial reaction. But calculating the conductivity from the
resistive component of the inductor impedance is not at all simple.